No need to enter a scrutinised value
While analysing the output of #13861 I stumbled over an unnecessary pessimisation in handling of scrutinised values. With words of Simon (from https://phabricator.haskell.org/D4267 with minor edits added):
Interesting. Yes, please make a ticket! (And transfer the info below into it.)
I think the issue is this. Given (the STG-ish code)
data Colour = Red | Green | Blue
f x = case x of y
Red -> Green
DEFAULT -> y(here y is the case binder) we can just return x rather than entering it in DEFAULT branch, because y will be fully evaluated and its pointer will be correctly tagged.
You absolutely can't check for an OccName of "wild"!! That is neither necessary nor sufficient :-).
Instead, check isEvaldUnfolding (idUnfolding y). See Note [Preserve evaluatedness] in CoreTidy.hs. And be sure to augment that note if you make the change.
I would expect perf benefits to be small on average, but it's simple to implement.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 8.2.2 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | #13861 |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |